In the field of mineral mining, hydraulically-operated telescopic props are used extensively to support the roofs of mine workings. Such props normally have an outer base member supported directly or indirectly on the floor of the working and one or more inner concentric members which extend and retract in relation to the outer member in telescopic fashion.
Various types of support assemblies used in mine workings, such as walking supports or frames and shield supports, all employ multi-or single stage Props. A typical two stage prop of simple design is described in German Patent Specification No. 2 032 455. A typical known prop has an outer base member and a first inner member provided with a piston which can be subjected to hydraulic force when a working chamber in the base member is charged with pressure fluid. A spring-biased non-return valve can be provided on the piston of the first member and a plunger can operate this valve to permit communication between the working chamber in the base member and a working chamber in the first member. It is also known in the art, to provide telescopic props with additional chambers which can be charged with pressure fluid to forceably retract the prop stages. This is the so-called `robbing aid`.
A general object of the present invention is to provide an improved form of telescopic prop.